Thermoplastic polyurethane (hereinafter TPU) resins are composed of easily movable long-chain units called soft segments and extremely crystalline units called hard segments and exhibit excellent physical properties ascribed to this characteristic structure, such as elasticity, elongation, mechanical strength, and abrasion resistance. With these physical properties taken into advantage, TPU resins have been used in a variety of fields as, for example, hoses, belts, wires, cables, pipes, soles, automotive interior and exterior trim, and various moldings.
A TPU resin is compounded with a flame retardant to be rendered flame-retardant. The use of a halogen flame retardant has the problem of harmful gas generation on combustion. It is known to use instead a nitrogen- and/or phosphorus-containing compound, such as melamine phosphate, melamine polyphosphate, or a condensed phosphoric ester, as a halogen-free flame retardant.
It is also proposed to use an intumescent phosphate flame retardant that swells on combustion to form a swollen surface layer, which prevents diffusion of a decomposition product and heat transfer thereby to achieve flame retardation. In order to further enhance the flame retardation effect, a combined use of these flame retardants with silicon oxide has also been proposed (see Patent Literatures 1 and 2 below).
The problem with the use of silicon oxide is that some types of silicon oxide bring about no improvement on flame retardation effect. Accordingly, a flame retardant aid that surely improves flame retardation effect has been sought.